High-power switches are typically fabricated using shunt-connected diodes. As known to those skilled in the art, shunt-connected diodes are preferred in high power switch applications because they provide high-isolation when in an ON state, minimize losses when in an OFF state, and because of their cost effectiveness as compared to mechanical switches.
In conventional applications, diodes may be mounted to substrates, such as printed circuit boards (PCBs), using a chip-on-board (COB) mounting scheme. Chip-On-Board is essentially the attachment of a die, in this case a diode, directly onto the surface of a PCB. Electrical contacts of the diode are wire bonded to conductors on the PCB. An encapsulant is then disposed onto the attached diode and wire bonds to protect them from mechanical damage. Unlike packaged diodes, as further discussed below, a COB-mounted diode does not suffer from packaging parasitics. There are, however, significant disadvantages associated with COB-mounted diodes. For example, since wire bonds are exposed during most of the COB assembly process, the wire bonds routinely become damaged, making the already expensive COB assembly process even costlier. Additionally, since the number of components that may be mounted near COB-mounted diodes is limited, relatively large PCBs are required. For these and other reasons, the industry has gone away from COB-mounted diodes and towards packaged diodes.
Packaged diodes, as their name implies, are diodes that are pre-packaged in an encapsulating material. Packaged diodes are much easier to handle, easier to manufacture without risk of damage, and thus, less expensive than their unpackaged counterparts. A typical packaged diode is illustrated in FIG. 1, wherein a PIN diode 100 is shown encapsulated in a low-cost, surface mount plastic package 110. This particular plastic diode package 110 is shown having three (3) leads 120 protruding therefrom, which are bent down and used to surface-mount the diode 100 onto a PC board (not shown).
As indicated above, packaged diodes have distinct advantages over to non-packaged, COB-mounted diodes. Packaged diodes, however, also have their own set of drawbacks. One of the most recognized drawbacks of packaged diodes is the existence of ‘package parasitics’. Package parasitics are unwanted electrical and mechanical attributes that result from the physical construction of the package itself. Once such unwanted parasitic is inductance. As known to those skilled in the art, ground inductance in shunt switching elements decreases the isolation in a radio frequency (RF) switch. In packaged PIN diodes, such as is illustrated in FIG. 1, there is a connection between the PIN diode 100 inside the package 110 and an exterior package termination (i.e., feed) 120. Since this connection is conductive, there will always be an inductance associated with the connection. This inductance limits the isolation of the packaged diode 100, particularly at high frequencies, which results in reduced overall performance.
Accordingly, it is desirable to have a method and apparatus for providing a low-parasitic, high isolation package structure, particularly for use in high power switch applications, which is easy and cost-effective to manufacture.